1. Field of the Invention
The present invention relates to low expansion ceramics, more particularly cordierite series dense, low expansion ceramics having excellent thermal shock resistance, being airtight and being heat resistant.
2. Related Art Statement
Recently, accompanying the rapid progress in industrial technologies, demand for industrial materials having superior heat resistance and thermal shock resistance have increased. Thermal shock resistance of ceramics is influenced by thermal expansion coefficient, thermal conductivity, strength, elastic modulus, Poisson's ratio, etc. of ceramics, by shape and size of products, and further by heating or cooling conditions of ceramics, namely, heat transfer rate of the ceramics.
It has been known that thermal shock resistance of ceramics is influenced largely by thermal expansion coefficient among the aforementioned factors, and largely dependent exclusively on thermal expansion coefficient especially when heat transfer rate of the ceramics is large. Therefore, it has been desired to develop low expansion ceramics having excellent thermal shock resistance.
Heretofore, cordierite has been known as a ceramic substance of relatively low expansion. However, it is generally difficult to produce a dense body by sintering. Particularly, when it is desired to produce low expansion cordierite of an average thermal expansion coefficient of not greater than 2.0.times.10.sup.-6 /.degree.C. in a temperature range of from ambient to 800.degree. C., it has been necessary to restrict the amount of impurities such as calcia, alkaline earths, potassium, sodium and the like which acts as a flux in producing the cordierite to a much minor amount, so that the resultant cordierite has a very small amount of glass phase and is very porous. Particularly, cordierite honeycomb structures, such as those of U.S. Pat. No. 4,295,892 to Matsuhisa et al, which have recently been widely used as substrates for catalysts for purifying exhaust gas of automobiles necessitate an average thermal expansion coefficient of not greater than 1.5.times.10.sup.-6 /.degree.C. in a temperature range of from ambient to 800.degree. C., so that a raw material such as talc, kaoline, alumina or the like raw material of a low impurity or flux content is used. As a result, open porosity of the produced cordierite is 25-45% at the minimum.
Accordingly, if such cordierite ceramics are used, for instance, as a honeycomb structural body for a regenerator type heat exchanger, the honeycomb body has such a large open porosity that the pores, particularly the communicated pores, on the partition walls which define the holes of the honeycomb structure, incurs leakage of fluids between a heating fluid and a heat recovery fluid from either side of the fluids. Hence, serious drawbacks occur that efficiency of the heat exchanger and efficiency of the total system wherein the heat exchanger is used becomes inferior. While, if such cordierite of high open porosity is used e.g. as a housing for a turbocharger or an exhaust manifold of an engine, a serious drawback occurs that the interior air of high pressure leaks to the exterior of the manifold etc. because of the high open porosity. Therefore, it has been desired to develop dense and low expansion cordierite ceramics having splendid thermal shock resistance.
Moreover, for structural materials which are exposed to such high temperatures, dimensional stability at high temperatures has been desired which is within .+-.0.05% in practical use.
Hitherto, for obtaining a dense cordierite ceramic, a method has been known wherein a batch of composition for preparation of cordierite is melted, molded and subjected to a crystallization treatment to obtain glass ceramics. For example, the report of Topping and Murthy described on Journal of the Canadian Ceramic Society, 46, 1977 proposed to substitute AlPO.sub.4 for SiO.sub.2 in cordierite in an amount of 20% by weight at the maximum. According to the report, a composition of main components of raw materials added with AlPO.sub.4 is melted and cooled to form cordierite glass, and reheated and cooled to produce cordierite crystals. The thus obtained cordierite is dense. However, the cordierite has a drawback of still large thermal expansion coefficient of 2.15.times.10.sup.-6 /.degree.C. at the minimum, because orientation of precipitated cordierite crystal phases was unable to be controlled.
Japanese patent application laid open Nos. 59-13,741 and 59-92,943 proposed crystallized glass bodies produced by preparing compositions of main components of raw materials containing Y.sub.2 O.sub.3 or ZnO added with B.sub.2 O.sub.3 and/or P.sub.2 O.sub.5, firing the compositions to produce crystallized glass components, pulverizing or grinding the crystallized glass components to form glass frits of 2-7 .mu.m, forming or molding the glass frits to a desired shape, and refiring the shaped glass frits to obtain the crystallized glass bodies which, however, has a drawback of large thermal expansion coefficients of 2.4-2.6.times.10.sup.-6 /.degree.C.
U.S. Pat. No. 3,885,977 "Anisotropic cordierite monolith" issued on May 27, 1975 to I. M. Lachman et al disclosed a low expansion cordierite ceramic wherein crystal phases of cordierite ceramics are oriented in a plane due to the use of planar clay or laminated clay in raw materials for the cordierite ceramics. The cordierite glass ceramic is dense, however, it has a drawback of still high thermal expansion coefficient of 2.0.times.10.sup.-6 /.degree.C. or more.
Moreover, the prior art is silent regarding mentioned dimensional stability when held at high temperatures for a long time.